By Brian Santo, contributing writer
Researchers around the world have been trying to create a metalens, a single, lightweight lens that can focus all of visible light. Incremental progress has been made in the last few years, but researchers at Harvard University appear to be the first to create a metalens that covers almost the entire visible spectrum, albeit still at the nano scale.
Different wavelengths of light travel at speeds through the glass of traditional lenses. The difference between infrared and ultraviolet is enough that the two extremes will have different foci, forcing camera manufacturers to compensate in order to avoid color distortion. The very best cameras can focus all visible light well enough to satisfy all but the most exacting professional photographers, but only with a series of lenses which, in combination, are both bulky and heavy.
Camera makers have a clear interest in being able to replace those combinations of traditional lenses with a fewer number of lenses, or better yet, a single lens with extraordinary color-capture qualities. Other applications that would benefit from such a thing include super-resolution imaging systems, microscopes, and spectroscopy. The expectation is that manufacturers of devices that cameras get built into, such as smartphones and virtual reality (VR) headsets, would also be greatly pleased to have a lightweight camera lens, and if the color integrity is better than what they can get now, that would be a bonus.
Researchers around the world have been pursuing a particular type of metalens that uses structures called nanofins to focus light. (Chinese researchers have published extensively on research in this area.) These metalenses are lightweight, planar (in contrast to traditional lenses that can be convex or concave), and have had good color integrity.
The team at Harvard reports in Nature Nanotechology that it has developed a metalens of a single layer of nanofins “whose thickness is on the order of the wavelength” that can focus light from 470 nm to 670 nm, a wider swath of the spectrum than anyone else has been able to achieve with a metalens thus far, claims Harvard. While that’s not quite the full visible spectrum, it’s missing the farther reaches of the infrared and ultraviolet.
The Harvard researcher writes that with “judicious design of nanofins on a surface, it is possible to simultaneously control the phase, group delay, and group delay dispersion of light, thereby achieving a transmissive achromatic metalens with large bandwidth.”
Harvard says that its metalens is easy to fabricate and cost-effective, and that it has already licensed the technology to a startup, which it did not identify, for commercial development.
The next step, reported the Harvard news service, is scaling up the size of the lens, with a goal of reaching 1 centimeter in diameter.
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